Conventional communications systems in which signals are transmitted through a cable have two principal types of signal interference. The first type of signal interference is an echo signal, in which a portion of a transmitted signal is reflected back toward the source of the signal. Such interference is caused by impedance mismatches associated with lumped elements of the system such as connectors and termination circuitry. The second type of signal interference is near end cross talk interference (NEXT) which is usually found in full-duplex bi-directional communications systems having a transmitter and a receiver coupled to a single wire of the cable and is generated across impedances in the cable and by the transmitter.
These echo/NEXT (E/N) interference signals are typically comprised of a few high energy echo signals and a large number of lower energy NEXT signals. Various cancellation techniques are currently used to eliminate these types of interference. See “The Bell System Technical Journal,” Vol. 59, No. 2, February 1980, pp. 149-159 and Data Communication Principles, R. R. Gitlin et al, pp 607625.
When E/N cancellation (E/NC) methods are implemented in hardware as integrated FIR filters, a negative image for each one of the plurality of echo/Next (E/N) signals is digitally created from the transmitted signal, which is then summed with a received signal. FIR filters employ a plurality of taps in a delay line, wherein each tap is coincident with a particular interference signal. Unique multiplicand coefficients associated with each tap provide weighting to the time-sampled signals, and the plurality of weighted samples are summed to attain the desired filtered cancellation signal.
Conventional systems and methods for digitally filtering signals in the manner described above suffer from significant drawbacks. Specifically, such systems and methods employ a single FIR filter IC for filtering both low amplitude and high amplitude signals. However, filtering high bit resolution signals requires wide data paths which occupy large surface areas and consume large amounts of power thereby increasing the fabrication and operating costs of such devices. Moreover, since the bit resolution, chip surface area, and power dissipation of all of the filters in a conventional FIR filter IC are identical and are designed to filter the highest amplitude E/N signal being filtered by the IC, using such devices to filter low amplitude signals is not cost effective or efficient from a performance standpoint.